In a usual photovoltaic system, a plurality of strings of photovoltaic modules are connected in parallel to achieve the desired electric power at a limited voltage. The individual strings have to be protected against the occurrence of reverse currents, i.e. of currents which have a reversed flow direction as compared to the currents generated in the proper operation of the photovoltaic modules of the respective string. In principle, a reverse current may only occur if the open terminal voltages or open-circuit voltages differ between the strings connected in parallel. Usually, this is avoided with strings of equal length, i.e. with strings built of a same number of solar modules, which are the normal case. Even with shading of the solar modules of an individual string, no significant reverse current occurs through this string, as the shading has no significant influence on the terminal voltage. Instead, the occurrence of reverse currents requires the presence of a fault, for example due to one or more solar modules of a string being shorted such that the open terminal voltage of the string clearly falls below the open terminal voltage of the strings connected in parallel thereto. Due to the internal diode structure of the solar modules, a reverse current may then flow through the faulty string which—depending on the current strength—may result in a strong rise in temperature up to a destruction of the solar modules of the strings. The short circuit of a photovoltaic module may be caused by a short circuit of one or more cells in the photovoltaic module or by a double ground fault of a photovoltaic module or of its cabling. Even if these faults are very unlikely, i.e. occur only very rarely in practice, arrangements have to be made as these faults hold a high damage and risk potential as all photovoltaic modules of the affected string may be damaged, and secondary damages may also occur due to local heating. Thus, there is the requirement to avoid the occurrence of reverse currents through individual strings that are connected in parallel in a photovoltaic system.
In the product Sunny String Monitor (http://download.sma.de/smaprosa/dateien/7356/SSM-UDE091221.pdf, product catalogue: Sunny Family 2009/2010 page 82) it is known for avoiding reverse currents to secure the individual strings within their parallel connection by string fuses which disconnect a faulty string from the other strings connected in parallel. Upon tripping of one string fuse due to a fault in the respective string, a warning is issued that identifies the faulty string. With the output voltages of the individual strings tending to increase, presently they are up to more than 1000 V, such string fuses are complex and correspondingly expensive. Further, the use of usual melting fuses is associated with a permanent loss of power. This loss of power also occurs, if, instead of fuses, diodes blocking the reverse currents are provided for the individual strings. For continuous monitoring the individual strings for smaller faults or power losses that point to a creeping or approaching breakdown, the product Sunny String Monitor comprises one current sensor per string. This current sensor determines the strength of the current generated by the respective string. For failure monitoring of the strings connected in parallel, the current flowing from all strings in operation are collectively evaluated, i.e. with regard to the relative values of the currents flowing from the individual strings.
The product Sunny String Monitor is provided for use together with an inverter for feeding electric energy from the strings into an AC grid. In the inverter, a controller operates an inverter bridge in such a way that a system voltage present between the bus lines, to which the strings are connected in parallel, is adjusted with regard to a maximum electric power of the strings. This procedure is known as MPP (Maximum Power Point) tracking, wherein the maximum electric power of the strings is usually adjusted within a range of generally possible system voltages, i.e. within the so-called MPP window.
Another prior art photovoltaic system is known from WO 2007/048421 A2. Here, a mechanical switch is additionally provided in one connection line of each string, which is opened to disconnect the respective string in case of occurrence of a reverse current from the bus lines. To protect this mechanical switch from damage due to a spark gap forming between its contacts, a semiconductor switch is provided between the bus lines to short them temporarily upon opening and also upon eventually closing the switch.